INDUSTRIAL AND ENGINEERING CHEMISTRY
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(37) Graham, T r a n s . R o y . Soc. London, 151, 183 (1861); J . Chem. SOC., 17, 318 (1864). (38) Hale, Translations of three German reports, Office of Rubber Reseive, Oct. 1, 1947. (39) Hall, Hauser, le Beau, Schmitt, and Talalay, IWD. ESG. CHEM., 36, 634 (1944). (40) Hardy, T r a n s . Rou. Soc. ( L o n d o n ) , 33,326 (1900). (41) Harkins, J . Chem. Phys., 13, 381 (1945); 14, 47 (1946). (42) Heidenreich, and Peck, J . Applied Phys., 14, 23 (1943). (43) Huggins, J . Chem. P h y s . , 9, 440 (1941). (44) Kern, Kunststofe,28, 257 (1938). (45) Kienle, IND. ENG.CHEM.,22, 590 (1930); 23, 1260 (1931). (46) Kienle and Petke, J . Am. Chem. Soc., 62,1053 (1940), (47) Kuhn, W., Kolloid Z . , 58, 2 (1934) : 60, 959 (1936). (48) Kuhn, W., and Kuhn, H., Hdv.Chim. A c t a , 26, 1394 (1943). (49) le Blanc, Kroger, and Kloz, Kolloid-Chem. Beihefte, 20, 356 (1925). (50) Levi and Giera, G a m . chim. ital , 67, 719 (1937). (51) Mark and Saito, Monatsh., 68, 237 (1936). (52) Marvel et al., private communication, December 1945. (53) Medalia and Kolthoff, Report t o Office of Rubber Reserve, Oct. 15, 1945. (54) Messenger, T r a n s . I n s t . Rubber I n d . , 9, 190 (1933). (55) Meyer, 2. physzk. Chem., B44,383 (1939); Helv. Chim. A c t a , 23, 1063 (1940). (56) Meyer and Ferri, Ibid., 19, 694 (1936). (57) Kaunton etal., Rept. PB 32,161 (1945). (58) Pierson, Report to Office of Rubber Reaeive, Jan. 11, 1945. (59) PioctoI, T r a n s . Faraday Soc., 16, 40 (1921). (60) Punimerer, Kautschuk, 2, 85 (1926). (61) Rodewald, Z . p h y s i k . Chem., 24, 193 (1897).
Vol. 41, No. 3
(62) Schoene, Green, Burns, and Vila, IA-D. ENC.CHEM.,38, 1246 (1946). (63) Schulz, 2. p h y s i k . Chem., A179, 321 (1937). (64) Schulz and Jirgensons, Ibid., B46, 105 (1940). (65) Signer and von Tavel, Helv. Chim. Acta, 26, 1972 (1943). (66) Smith and Holt, B u r . Standards J . Research, 13, 465 (1934). (67) Smith and Saylor, Ibid., 13, 453 (1934). (68) Spence and Ferry, J . Soc. Chem. I n d . , 58, 345 (1939). (69) Stamberger, K a u t s c h u k , 7, 182 (1931). (70) Staudinger. Ber., 59, 3019 (1926). (71) I b i d . , 62, 2893 (1929). (72) Staudinger and Heuer, Z.physik. Chem., A171, 140 (1934). (73) Staudinger, Heuer, and Husemann, T r a n s , Faraday Soc., 32, 323 (1936). (74) Staudinger and Hoenel-Ininiexidorfer, J . makromol. Chem., 1, 185 (1943). (76) Stevens, I n d i a Rubber J . , 108, 324 (1945). (76) Stockmayer, J . Chem. Phys., 11, 45 (1943). (77) I b i d . , 12, 125 (1944) ; chapter in "High Polymers," cd. by Twiss; New York, Reinhold Publishing Corp., 1944. (78) Stockmayer and Jacobson, J . Chem. Ph,ys., 11, 393 (1943). (79) Walling, J . Am. Chem. S O C . ,67, 441 (1945). (80) Whitby, Evans, and Pasternack, Trans. Faraday Soc., 38, 269 (1942). (81) White, Ebers, Shriver, and Breck, IND.ESG. CHEM.,37, 770 (1946). (82) Williams and Wyckoff, Science, 101, 594 (1945). RECEIVED November 11, 1946. Part of research program on synthetic rubber, sponsored b y the Office of Rubber Reserve, Reconstruction Finance Corp.
Surface Areas of Quicklimes H. R . STALEY AXD S. H. GREENFELI) Mussuchusetts Institute of Technology, Cambridge, Muss, T h e calcining characteristics of a high-calcium limestone were studied in the residual atmosphere of a muffle furnace. The change of the total surface areas of quiclrlimes with temperature was determined as was the minimum time of burning at various temperatures. The degree of completeness of burning was obtained by means of carbon dioxidk analyses of quicklimes. In general, once carbon dioxide has been liberated completely the surface area drops with increased time of burning. I t decreases from 2.01 square meters per gram for 1 hour at 1800' F. to 0.15 square meter per gram for 16 hours at 2400 F.
M
A S Y investigators both in this country and abroad have studied the decomposition of calcium carbonate. Com-
plete vapor pressure curves have been determined for limestones of varying purity by Tamaru, Shiomi, and Adachi ( 2 5 ) ; Matsui, Bito, Murayama, and Xadono ( 1 8 ) ; Southard and Royster (21); Hackspill ( 9 ' ; and Whiting (H),with some variations, The primary and secondary dissociation temperatures have also been determined, again with conqiderable variation, by Conley ( 7 ); Rito, Aoyama, and Matsui (3, 4); Zimens (28); and others. Depending upon the degree of purity of the stone and accuracy of measurement of the investigator, the temperature of primary dissociation varied from 900 O ( 7 ) t o 930 O C. (5); the secondary, from 894" (21) to 915' C . (3). The reaction rate has been found by several to be of the first order, proceeding only a t the interface between the calcium carbonate and calcium oxide (8, I S , 17). Others derived fractional rates (Id),whil (13) found the surface feaction t o be first action of zero order, and fractional orders ruling ip between. Conley ( 7 ) ,Splichal, Skramovsky, and Gall ( B )and , Krustinsons (15) found the reaction rate t o be a function of particle size,
as v, ell as temperature, carbon dioxide presauIe, and impurities Zawadski (BY) and Furnas ( 8 ) showed that the reartion started as nuclei on the surface of the limeqtone and spread in ~ i d c n l n g areas, always taking place in a narrow zone between the calcium carbonate and calcium oxide. Linzell (16) and his eo-workeis found that there is a minimum burning temperature below which no measurable carbon dioxide Tyould be evolved, and that that temperature was a function of the limestone used and the partial presqure oi carbon dioxidt, i n the atmosphere. They also found that there is a small residual carbon dioxide content even in the most severely calcined quicklimes. While all this work was done on the kinetics and thermodynamics of quicklime production, other M orkers began to recognize that quicklimes produced under dlfferent conditions had vastly dlfferent properties. Orton and Peppell (19) reporkd that quicklime produced a t 1472" F., had a denyity of 2.69, while that produced at 2012" F. had a density of 3 37. [Azbe ( 1 ) reported densities of from 1.46 t o 300 as the time and temperature of burning increased 9 The time of burning wai: not Ieported, but it must be assumed that the authors mere dealing wlth completely decarbonated materials. I n 1926, Haslam and Hermann (11) reported an optimum time and temperature of burning for two quicklimes, as evaluated by such methods as settling rates, pladicity figure, and acid reactlon, all of which involved slaking the quicklimes. However, it has been pointed out (23) that slaking conditions may modify th? properties of the final hydrate more than burning conditions. Ray and Mathera (20) evaluated their results in a similar manner t n o years later Most of the properties measured to evaluate the quiclclimes were, in reallty, secondary measurements of surface area, but before 1938 no suitable means wa? available for actual measurement of the surface a r m of finely divided, porous materials. In
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1949
I
that year Brunauer, Emmett, and Teller (6) derived an equation, based on kinetic theory, by which the surface area can be calculated from low temperature, low pressure gas adsorption.
As V , P , and PO are measured directly, V , and c may be obtained by plotting the experimental data as P/V(Po P ) VJ. PIP0 and measuring the slope, c l/V,c, and intercept l/Vmc. The surface area, for measurements at liquid nitrogen temperature (-195.5" C.) is then merely
-
-
S = 4.37Vmm2/g. (6,6 )
(2)
for 1. cc. of gas (S.T.P.) occupies 4.37 squarz meters of area H hen spread out in a monolayer a t -195.5' C., liquid packing being assumed. The quicklimes in this investigation were evaluated by means of the method of Brunauer, Emmett, and Teller (si. Figure 1 is a correlation of the variation of the surface areas of the quicklimes with the temperature of burning a t various constant times, and Figure 2 is of the variation of surface area with time a t various constant temperatures. As some of the quicklimes were not completely free from carbon dioxide, the carbon dioxide content was determined by conventional means (IO) to ascertain the minimum time of burning. MATERIALS AND APPARATUS
A high-calcium Missouri limeqtone of the following average analysis was used: Calcium carbonate Magnesium carbonate Silicon dioxide
R203
Not analyzed
98.5%
0.6%
0.5% 0.4%
0.1%
For determining the dead space in the surface area measurements, medical grade helium was used, after passage through a liquid nitrogen charcoal trap. Water-pumped nitrogen of 99.85% purity was, in earlier runs, taken into the apparatus through a dry ice-charcoal trap, but as the absence of the trap produced no change in the results, it was omitted during these runs. Only chemically pure reagents were used in the carbon dioxide determinations.
521,
Tho limestones were burned in a muffle-type globar furnace in which the silicon carbide quffle TIME OF BURNING reportedly maintains a uniform int$rior temperature within +20° F. at 2400" F. An electronic controller was usrd to keep the furnace temperature nithin *20" F. of the setting. The surface area apparatuq is similar to that used by Beebe, Biscoe, Smith, and Wendell (g), described in a previous paper ( 2 3 ) . TIME OF BURNING. HOURS The carbon dioxide content of the quicklimes was determined'in as lightly modified form of the apparatus shown in "Talbot's quantitative Analysin" (IO). The sample flask has a water-cooled reflux condenser to return most of the water vapor and hydrochloric acid. This condenser is followed by an absorption bulb filled with a saturated solution of silver sulfate in dichromic acid. Any of the oxides of sulfur and much of the remaining hydrochloric acid and water vapor are removed here. The next bulb, ,containing anhydrous copper sulfate, found more satisfactory than carbon dioxide-saturated calcium chloride, removes the last traces of water vapor. Finally, the carbon dioxide is absorbed in a bulb containing Ascarite. The analysis procedure was the same as that of Talbot (IO). Results obtained on this modified apparatus were reproducible, within 1% of their values. BURNlNG PROCEDURE
When the calcining furnace was a t the desired temperature, its insulated door was removed and approximately 3.5 pounds of 0.75- to 1-inch pebbles of calcium carbonate were spread over the floor of the furnace in order to permit uniform heating. The door was replaced within 30 seconds. The presence of this large mass of cold stone reduced the furnace temperature between 100" and 200" F., but the setting temperature was regained within a few minutes. This initial temperature drop made i t necessary t o choose some arbitrary initial time, which time was chosen as halfway between the closing of the furnace door and the attainment of the desired temperature. At the beginning of each run the atmosphere of the furnace was essentially that of the laboratory, but as the limestone decomposed, i t enriched the furnace gas composition with carbon dioxide. After decomposition was completed, the carbon dioxide slowly diffused out until the atmosphere was again that of the room. Upon completion of the burning operation, the door was again removed, and the lumps of calcium oxide were raked into a pan, where they were immediately crushed to -8-mesh and stored in a suitable container until needed. These initial and final periods at elevated temperatures other than the specified temperature introduced some slight error in both the times and temperatures of burfling. After the quicklime had cooled to room temperature, a representative sample of approximately 20 grams was weighed into a sample bulb and sealed to the surface area apparatus. The surface area determination was made in the usual way-that is, four values of the volume adsorbed were obtained in the relative pressure range of 0.05 to 0.25 and treated according to the method of Brunauer, Emmett, and Teller (6). I n cases where incomplete burning was suspected, carbon dioxide determinations were made following the procedure of Talbot (IO).
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
522 TABLE I.
SLZlFACB: &4RE.4S AXD C A R B O S r)IOXIDE h i A L Y S E S
--. Sample
LM-1
LM-1
LM-1 LM-lab LM-1 LM-1
Temp,, O F. le00 1700
---
Time, Hours-----.
a
Surfacg arena, fiq.m./g-
,
::
.,,
0.’.54 0.80 0.57 .. ,
n
90 0.94 ,
.,
Time, HoursCarbon dioxide,
1.58
...
...
34 85
...
17.60
0 . 9 6 1. O O 0 11 .,. 1800 2,Ol 1’.46 l’.’lO 0 9 1 0 . 8 6 2 48 1 . 4 4 0 . 8 5 0.58 0 . 4 6 0.35 2000 ... 0 . 6 3 , . . 0 . 4 2 0 . 2 9 0.22 2200 .. LM-1 ... 0.41 . . , ... ... Lhl-1 0.’1‘9 0.40 .. ... .,. LM-la b 0 . 3 6 0 . 2 6 0.2Y ,.. 2400 LM-1 ,,, . , . 0 . 2 9 0.’l$ 0 . 1 5 LM- 1 ab a Limestone, ground t o -8-mesh, had a jurfacu area of 0.37 s q . m , / g , 5 LM-la is from n different shipment of stone from s a m e q u a r r y
.
76
zi.ns 0.26
...
... ...
Minimum Time for Complete 1G Burning, Hr. B ,60 > 16 ... 6
-
..,
2
... ...
il 61
...
... .,. ...
...
I
.
Vol. 41, No. 3
were used commercially, burning could be completed at lower temperatures, as far as the removal of the rarbon dioxide is concerned, but tho increase in surface area would not be as indicated by simple extrapolation of the presented data. (Experimentally, with finely divided limestone, completely burned quicklime has been obtained as rapidly as 2 hours a t 1450” F.)
,
6 1
NOMENCLATURE
c = heat constant = k e ( E i - - E L ) / R T
B
= average heat of adsorption in the
hist layer
DISCUSSION OF R E S U L T S
Figure 1 shows the variation of the surface area of thc yuicklimes with the temperature of burning at constant tinics of burning. From this correlation can be seen that n.hile the surface area is very high for 1 hour’s burning a t 1800” F., indicating complete burning, at least 6 hour8 are required a t 1700’ I?. The carbon dioxide analyses shouf, in Table I, that the surface area begins to fall before the carbon dioxide has been completely eliminated. The surface area continues to drop on prolonged heating at any temperature, or as the temperature is increased for any given time of burning. (Commercially, any quicklime of less than 2% carbon dioxide is considered completely calcined. Many of t,hese arc: overburned, but become partially recarbonated while being cooled. Surface areas of several commercial quicklimes varied from 1.43 to 2.08.) Considering Figure 2, it can be seen that under the burning conditions employed the quicklimes produced a t 1700” F. are unlike the others. This mild temperature condition, even at as long a time as 16 hours, doe6 not produce the surface area that would be expected from the trend of the curves in both figures. Of course, the initial rising portion of the curve for the 1700” F. burning is a clear indication of the incompleteness of burning and its approach to completion with time. But this trend would indicate that even a t 16 hours the burning is not complete. However, carbon dioxide analyses show it to be complete at 6 hours. This apparent inconsistency points to the fact that dccoinposition probably is not the only factor contributory to the increase in surface area. As the temperature is too low for any compound formation between the calcium oxide and its impurkies of magnesium, aluminum, and iron oxides, the only other alternative explanat,ion is the orientation of the crystallites of calcium oxide to the cubic form from the hexagonal form of the calcium carbonate. At t,he higher temperatures this orientation probably is as rapid as the carbon dioxide liberation, but belov 1800” F. it. occurs more slorvly. As the temperature increases for any given time of burniiig the surface area decreases rapidly. This decrease is acconipanied by a large increase in apparent density (1, 19), which indicates a compacting of the calcium oxide crystals and shrinkage of the particles. Further evidence of this shrinkage is the elimination of the small pores in the more severely calcined materials ( 2 4 ) . Figure 2 shows that initially the time of burning is important, for the surface area drops off rapidly (after complete carbon dioxide elimination) for the first 6 hours. Beyond 6 hours there are only slight decreases in the surface areas wit,h time a t a specific temperature. However, there are still large decreases as the temperature is increased, even for the 16-hour ca1cinat)ioIis. I n considering these data it must be remembered that they were obtained for 0.75- to 1-inch lumps of cdcium carbonate in a muffle furnace. The atmosphere essentially carbon dioxide, much different from the atmosphere iri a commercial kiln, which is rarely above 30% carbon dioxide. If the same sized stones
EL = heat of liquefartiori
number of layers of adsorbate on surface of adsorbent, or radius, pore size (in adsorbate molecular diameters), limiting adqorption P = pressure PO = vapor pressure at the teinperatule of the adsorbent R = gas constant S = Turface area T = absolute temperature V = volum? adsorbed (S.T.P.) Vm = volume adsorbed (S.T.P.) in a complete monolayer Y = P’P0 = relative pressure
n
=
BIBLIOGRAPHY
&be, V. J., Rock Products, 42, 40-1 (1939). Beebe, R., Biscoe, J., Smith, W., aud Wendell, C., J . A.m. Chern. Sot., 69, 95-101 (1947). Bito, K., Aoyama, K., and Matsui, M., J . SOC.Chem. 17bd. J a p a n , 35, Suppl. Binding, 191-5 (1932). Ibid., 36, 152-3 (1933). Brunauer, S., “The Adsorption of Gases and Vapom,” Princeton, Princeton TJnivemity Press, 1945. Brunauer, S., Emmett, P., and Teller, E., b. A m . Chem. Soc., 60, 309 (1938). Conley, J. E., Am. Inst. Jfi?Li?ay M e t . Engrs., Tech. Pub. 1037 (1939). Furnas, C. C., IND.E m . C/HE:M., 23, 534-38 (1931). Hackspill, L., Compt. rend., 203, 1261-3 (1936). Hamilton, L., and Simpaon, S.,“Talbot’s Quantitative Analysis,” pp. 243-8, New York, Maomillan Co., 1941. Haslam, R. T., and Hermann, E. C., IND.ENC.CHmr., 18, 960 (1926).
Hodgman. C. D., Ed., “Handbook of Chemistry and PIiy&x,” 27th ed., p. 1777, Cleveland, Chem. Rubber Pub. Go., 1944. Huttig and Kappel, Angaw. Chem., 53, 57-9 (1940). Khomyakov. K. G., Yavorovskaya, S. F., and Arhusov, T’.
